As known in the art, the hydrogenation reaction of phenol in the presence of a palladium catalyst gives a mixture including cyclohexanone and cyclohexanol (Patent Literatures 1 and 2). Cyclohexanone is used as a raw material in the production of caprolactam, and cyclohexanol mixed in the cyclohexanone is an undesired impurity. For example, cyclohexanol may be converted into cyclohexanone by dehydrogenation with a copper oxide/zinc oxide catalyst (Patent Literature 3). However, additional costs are incurred in order to separate a cyclohexanone/cyclohexanol mixture obtained by the hydrogenation reaction of phenol into cyclohexanone (boiling point 156.4° C.) and cyclohexanol (boiling point 161.1° C.) and also to dehydrogenate cyclohexanol. In consideration of these costs, the occurrence of cyclohexanol as a byproduct in the hydrogenation reaction of phenol is desirably suppressed to the minimum.
Because cyclohexanol is formed by the hydrogenation reaction of cyclohexanone, the amount of byproduct cyclohexanol may be reduced by decreasing the rate of the conversion of phenol. However, cyclohexanone and phenol, and cyclohexanol and phenol form maximum-boiling azeotropes and therefore decreasing the conversion rate gives rise to another economic problem that a significantly high cost is incurred to separate cyclohexanone and cyclohexanol as the products from the unreacted phenol. Thus, phenol is desirably converted at as high a conversion rate as possible.
Further, the hydrogenation reaction of phenol produces high-boiling byproducts based on cyclohexylcyclohexanone, in addition to cyclohexanol. These byproducts are generally difficult to convert to cyclohexanone by an affordable method in contrast to the dehydrogenation of cyclohexanol into cyclohexanone. Thus, an increase in the amount of cyclohexylcyclohexanone byproduct leads to a decrease in the yield of cyclohexanone, resulting in poor economic efficiency.
From an industrial viewpoint, the satisfaction of both high phenol conversion rate and high cyclohexanone selectivity is an important key to the economically advantageous production of cyclohexanone. The production of cyclohexanone by the hydrogenation reaction of phenol in a gas phase is generally performed by passing a mixture gas of phenol and hydrogen through a palladium catalyst supported on an alumina carrier. The process, however, is not applicable to an industrial scale because the catalyst is frequently deactivated in a short time.
To address the above problem, for example, it is reported that a palladium catalyst which is supported on a carrier prepared by mixing alumina with an alkaline earth metal hydroxide is less prone to deactivation and shows enhanced cyclohexanone selectivity as compared to when γ-alumina is used as the carrier (Patent Literature 4). However, the carrier made by this method has a defect in that mechanical strength is generally low. Further, the hydrogenation reaction of phenol involves a large excess of hydrogen and the catalytic activity is below the level required for use on an industrial scale.
To avoid this problem, the use of alumina spinel as a carrier is reported (Patent Literature 5). This approach realizes high mechanical strength and sustained catalytic activity. However, this method entails more complicated carrier production steps and involves more expensive raw materials than when usual alumina is used. These facts inevitably raise the carrier production cost.
From the viewpoints described above, the development of a low-cost and simple process which can produce cyclohexanone with a high yield is desired.